A New Heavy-Ion Facility at Chalk River - ACS Publications

63 Recent Developments at T R I S T A N Nuclear Structure Studies of Neutron-Rich Nuclei R. L. Gill Brookhaven National Laboratory, Upton, NY 11973

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch063

The nuclear physics program at the fission product mass separator, TRISTAN, has greatly expanded, both in the types of experiments possible and in the range of nuclei available. Surface ionization, FEBIAD, high-temperature thermal, high-temperature plasma, and negative surface ionization ion sources are routinely available. Experi mental facilities developed to further expand the capa bilities of TRISTAN include a superconducting magnet for g-factor and Q measurements, a windowless Si(Li) detector for conversion electron measurements, and a colinear fast-beam dye laser system for hyperfine inter action studies. This combination of ion sources, experimental apparatus, and the long running time avail able at a reactor makes TRISTAN a powerful tool for nuclear structure studies of neutron-rich nuclei. The effect of these developments on the nuclear physics program at TRISTAN will be discussed and recent results from some of these facilities will be presented. β

The TRISTAN F a c i l i t y A schematic layout of the TRISTAN ISOL f a c i l i t y 1. High

A neutron

beam of about

Flux Beam Reactor

3xl0

1 0

2

n /cm /sec tn

i s shown i n F i g .

i s provided by the 60 MW

at Brookhaven N a t i o n a l Laboratory.

Targets which

235

t y p i c a l l y contain 5g

U i n s i d e an i o n source are positioned i n the neutron

beam to provide intense beams of s h o r t - l i v e d n e u t r o n - r i c h n u c l e i produced by fission. used,

To provide f o r maximum v e r s a t i l i t y ,

each

of which i s best

suited

a v a r i e t y of i o n sources are

to producing c e r t a i n

elements

GIL85, PI084]. 0097-6156/ 86/ 0324-0420$06.00/ 0 © 1986 American Chemical Society

Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

[SHM83,

Nuclear Structure Studies of Neutron-Rich Nuclei

GILL


63.

421

Fig.

2.

Schematic diagram of the high temperature plasma i o n source.

High Temperature Plasma Ion Source The

most

plasma i o n source.

recently

developed

i o n source

The source i s shown i n F i g . 2.

i s the high

temperature

I t i s constructed i n a

fashion s i m i l a r to the thermal and negative i o n sources i n that these three sources

share

shields. fied

a

common

design

f o r the

t a r g e t , heating

owing to s t a n d a r d i z a t i o n of the designs.

that t h i s

filamental

heat

Thus, mass production and assembly of the sources has been s i m p l i Off-line

i o n source e x h i b i t s i o n i z a t i o n e f f i c i e n c i e s

studies have shown

of up to 60% f o r Kr


422

NUCLEI OFF THE LINE OF STABILITY

and Xe i n the low pressure l i m i t . elements ture

This source produces

the same v a r i e t y of

as the FEBIAD source, but due to the l a r g e r target, higher tempera

and

high

ionization

efficiency,

the

intensities

of

the

radioactive

beams are s i g n i f i c a n t l y higher than with the FEBIAD source. Experimental F a c i l i t i e s In F i g . 1, the f a c i l i t i e s a v a i l a b l e at each of the f i v e beam ports at TRISTAN are l a b e l e d . ences t h e r e i n . become

fully

The

Si(Li)


facilities

o p e r a t i o n a l and

f a c i l i t i e s w i l l now

electron

The

widely

used

in

the

past

year.

Detector.

A

windowless

Si(Li)

i n a cryostat

detector

These

two

be moved to w i t h i n 5 mm

f o r conversion

with a compressable

to a vacuum chamber and moving tape c o l l e c t o r .

radioactive

refer

be described i n some d e t a i l .

spectroscopy i s housed

connected

are described i n [GIL85] and

S i ( L i ) detector and superconducting magnet systems have

of the surface of an aluminized mylar

ion beams are implanted.

The

tape system

that the ion beam d e p o s i t i o n point i s d i r e c t l y viewed located i n an area shielded from the d e t e c t o r .

bellows

The detector can tape on which

can be configured so by the detector or i s

In the l a t t e r

configuration,

the tape must be moved before the S i ( L i ) detector can view the source. problem

encountered

neutron-rich desired gating

nuclei

peaks.

i s used

doing

i s the

This

techniques.

response)

when

conversion

tendency

interference A

thin

of

can

plastic

to suppress

the

be

3

suppressed

by

to

very

Si ( L i )

with

obscure

employing

(with

i n the

One

spectroscopy

continuum

scintillator

3 signal

the

electron

the

various

low

γ-ray

detector.

The

system i s designed such that a standard Ge detector cryostat also views the same source as the S i ( L i ) d e t e c t o r .

Thus, γ-ray e l e c t r o n and x-ray e l e c t r o n

coincidences can also be employed to enhance the e l e c t r o n s i g n a l , as w e l l as provide f o r element i d e n t i f i c a t i o n . bilities, it

In a d d i t i o n to these coincidence capa

e l e c t r o n and γ-ray s i n g l e s can be simultaneously acquired, making

possible

to measure

internal

conversion c o e f f i c i e n t s .

been used to i d e n t i f y weakly populated 0 Superconducting magnet system,

Magnet.

manufactured

f o r g-f a c t o r and

A

by Oxford

Q3 measurements.

s e c t i o n room temperature for

the

tape

from

states i n

96

split-pole, Instruments,

The

Τ at 108 A at 4°K with inhomogeneity

,

+

>

magnet has a f i e l d 2

tape

collector.

The

has

superconducting

i s available

of +

nucleus

where

N

-

Ν /Ν ,

a

observed

straight only

± 0.05)

can be described

t

f o r isotopes

ν

g(2+!) » (0.24

for

ltf6

Ν

of

line

f o r those

π

+

N .

If

v

Ba-Dy

by

are used

i s obtained

with

isotopes expected

the

deviations

are i n a d i r e c t i o n

I f only those

± 0.04 and g of g

nuclei

i n this

used

v

*

+

g(2 i)

significant to e x h i b i t

a

shell

state i n N

gπNπ/ t values

t

are accepted

as being

+

of

versus

v

deviations a truncated

being proton

A d d i t i o n a l l y , a l l of the smaller

proton

valence

isotopes with N>90 are included i n the f i t g I f these e m p i r i c a l l y constant

over

region, then the experimentally determined

ir

- 0.63

determined

a wide range of g - f a c t o r s can be (Ν

π

i n IBA-2

The r e s u l t s of t h i s a n a l y s i s are shown i n F i g . 3 f o r Ba, Ce,

and Sm i s o t o p e s .

major

+

N /N

as a measure of the e f f e c t i v e number of valence protons

formalism). Nd,

and g

ir

+

g(2 i)

experimental

to p l o t

indicates

- 0.05 ± 0.05 i s obtained.

v

values

that

The

C e and g ( 2 x ) - (0.37 ±

[SAM84]

valence space due to the s u b s h e l l c l o s u r e at Z=64.

space.

technique

+

In IBA-2 formalism, the g - f a c t o r of the f i r s t 2

g(2 i) π

cascade) are measured at an a p p r o p r i

down.

[W0L83] since t h i s technique e l i m i n a t e s most systematic e r r o r s .

r e s u l t s obtained are: 0.06)

+

The dashed

closures from

line

shows the value of Ν

Z-50-82 (Z-82) and from

π

expected f o r

Z-50-64 (Z-64) and the

s o l i d l i n e i n d i c a t e s the region of t r a n s i t i o n between s h e l l s

[GIL82],

figure

a tendency of

the

Ν

shows not only π

change

the d i s t i n c t

to s h i f t

to lower

change neutron

i n Ν , but also π

number

as Ζ i n c r e a s e s .


The

This

63.

Nuclear Structure Studies of Neutron-Rich Nuclei

GILL

425

T

Ba

6

Ce Z = 82

4

Ζ =82 Ζ = 64

2

Z = 64

Nd


6

Ζ =82

4 Z = 64

2

-È

L

84

86

_l_

88

_L

90

92

84

86

88

90

92

Ν Fig. and

3.

Ν

β ί π

*

deduced

f o r isotones of Ba, Ce, Nd, and Sm.

Sm data are from [WOL83] and references t h e r e i n .

The Ba,

Nd,

The Ce data i s from

t h i s work.

effect

i s primarily

becoming

possible

neutron-proton are occupying

due to e a r l i e r

as higher

orbitals

occupation are f i l l e d

of the with

hn/2

increasing

i n t e r a c t i o n can then operate to the f u l l e s t the ho/2 o r b i t a l ) r e s u l t i n g

ïï

extent

orbital Z.

i n a deformed nucleus having the

lowest energy. Acknowledgment Research supported under contract DE-AC02-76CH00016 with the U.S. DOE.

References [SHM83] M. Shmid et al., NIM 211 287 (1983). [GIL85] R. L. Gill and A. Piotrowski, NIM 234 213 (1985). [PIO84] A. Piotrowski et al., NIM 224 1 (1984). [WOL83] A. Wolf et al., Phys. Lett. 123B 165 (1983). [SAM84] M. Sambataro et al., Nucl. Phys. A423 333 (1984). [GIL82] R. L. Gill et al., Phys. Lett. 118B 251 (1982). RECEIVED

The

(neutrons

July 14, 1986


A New Heavy-Ion Facility at Chalk River - ACS Publications

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